1. Field of the Invention
The present invention pertains to transparent glazing units, and more particularly to such glazing units having transparent coatings adapted to reduce solar transmittance and increase solar reflection for minimizing solar heat gain through the glazing unit. The coatings include electrically conducting components which permit optional heating of the glazing units.
2. Description of the Prior Art
It has long been recognized that transmission of solar energy through glass results in significant heat gain within glazed enclosures. In an effort to reduce this transmission of solar energy, and thus heat gain, so-called heat-absorbing glasses were developed wherein the solar energy outside of a particular bandwidth is absorbed by predetermined elements in the glass and reradiated as heat. With the advent of air conditioning, particularly in automobiles, reducing transmission of solar energy through the glazing units became increasingly important in order to minimize the load on the air conditioning systems.
In order to comply with Federal standards, it is important that automotive glass have a low total solar transmission, with relatively high transmission in the visible spectrum. The standards specify that automotive glass have Illuminant "A" Transmittance of at least seventy percent in the visible spectrum. Such a transmission level in the visible spectrum insures that drivers will be able to safely view objects outside the vehicle under all light conditions, including headlights of oncoming vehicles. To be deemed a heat absorbing glass, the total solar transmittance must not exceed fifty percent at the same glass thickness.
Heat absorbing glass is conventionally made by incorporating into the glass batch materials, additional iron so as to maintain in the glass following melting, refining and forming, an increased level of iron in the ferrous state. Such glass has a slightly greenish appearance. While such heat absorbing glass does exhibit lower solar transmission, it is not entirely effective in reducing heat gain behind the glazing unit. Thus, as the unit absorbs energy it becomes heated and consequently reradiates the absorbed energy both to the inside and the outside of the enclosed space. Energy reradiated to the enclosed space, as to a vehicle or building interior, becomes a solar heat gain which is added to the direct solar heat gain resulting from solar energy passing through the glass unabsorbed. As the amount of glass employed in building structures and vehicles increases in response to aesthetic and styling dictates, the solar heat gain also increases, resulting in increased air conditioning loads.
More recent efforts have been concentrated on reducing heat gain by increasing total solar reflection of the glass. Radiation from the sun which is reflected and not absorbed, does not result in a heat gain behind the glazing unit since it is neither transmitted through the unit nor absorbed thereby to be transformed into heat. To that end, it has been previously proposed to provide glazing units with various forms of filters exhibiting improved characteristics of solar reflection. The filters generally provide for high transmission over the visible portion of the solar spectrum and high reflection over the infrared portion. Recent filters of this nature for glazing units, and particularly for multi-layer automotive windshields, have included a stack of coatings comprising two or more materials deposited in alternate layers on the surface of a substrate, either the glass itself or a polyester base which is, in turn, laminated to a surface of the glazing unit. Generally the composition and the thickness of the layers are selected, and the stack is designed, to minimize the heat energy absorbed inside the glazed enclosure such as, for example, an automobile, by reducing solar transmittance and/or increasing solar reflection. However, it has been found that with the combinations of layer compositions and thicknesses employed heretofore, the glazing units exhibit an objectionable visible reflectance and color which often clash with the setting in which they are utilized. There has not been available a glazing unit having a multi-layer coating possessing the desired features of reduced visible reflectance, low total solar transmittance, high solar reflection and compatible color.